1. Field of the Invention
This invention relates to a frequency mixing crystal suitable for use in optical frequency harmonic generators, and other instruments.
More particularly, it relates to a frequency mixing crystal for converting laser emissions in the infrared spectrum into ultraviolet or visible light and vice versa.
Harmonic generation of laser beams using various frequency mixing crystals is known. Harmonic generating apparatus is particularly useful for high-intensity laser systems where intense optical power is easily and relatively efficiently obtainable for certain longer wavelengths but not at shorter wavelengths. For example, neodymium-glass lasers produce wavelengths in the one micron infrared range. Direct production of green in the one-half micron range or ultraviolet in the one-third micron range is not easily or efficiently accomplished. Therefore, harmonic generators which convert laser beams from those which have wavelengths in the infrared region to those in the visible light region perform an important, useful function.
2. Prior Art
One of the earliest harmonic generation experiments was performed by Franken et al. in 1961, shortly after the invention of the laser. See Franken, P.A., Hill, A.E., Peters, C.W., and Weinreich, G., "Generation of Optical Harmonics," Phys. Rev. Lett., 118 (1961). In that experiment, a pulse from a ruby laser at 694.3 nm was focused into a quartz crystal and a very small amount of energy at the second-harmonic frequency 347.15 nm was obtained.
To be practically useful, much greater conversion efficiencies were required, and a technique called phase-matching was developed to improve conversion efficiency. Phase-matching is required because the fundamental wave travels through the material at a different velocity than does the harmonic wave generated by the fundamental due to normal dispersion. If the proper phase between the fundamental and harmonic waves is not maintained, the second-harmonic waves generated at different points in the material destructively interfere resulting in poor conversion efficiency.
Second and third-harmonic waves are obtained with the use of so called non-linear frequency mixing crystals. See "Optical Waves in Crystals," Yariv et al., John Wiley & Sons, 1984, Chapter 12 in particular. A non-linear crystal is one in which the output wave does not depend purely on the input wave.
Commonly used for harmonic or frequency conversion are birefringent crystals, such as potassium dihydrogen phosphate (KDP). KDP has the characteristic that, for a fundamental input wave which is a linearly polarized ordinary wave, the resulting second-harmonic is an extraordinary wave. If the crystal is oriented so that the index of refraction of a fundamental ordinary wave equals, or is matched to, the index of refraction of the second-harmonic wave, the various second-harmonic waves produced as the fundamental wave propagates through the crystal will constructively interfere and greatly improve the conversion efficiency of the crystal.
Frequency conversion of the 1.053 micrometer output of high-power neodymium-glass lasers recently has received much interest at the Lawrence Livermore National Laboratory. Irradiation of laser fusion targets by shorter wavelengths provides significant advantages. Production of third-harmonic waves at 0.35 micrometers from 1.053 micrometer, high-intensity waves were reported by Seka et al. in "Demonstration of High Efficiency Third-Harmonic Conversion of High-Power Nd-Glass Laser Radiation," Optics Communications, Vol. 34, No. 3, p. 469 (1980). The apparatus incorporated spatially separated Type II KDP crystals.
The Nova Laser Fusion Project at the Lawrence Livermore National Laboratory requires large-aperture, high-fluence lasers beams at the second and third-harmonics of the 1.053 micrometer wavelength Nd-glass lasers.
The KDP family of frequency mixing crystals, described in U.S. Pat. No. 3,949,323, can be used in conjunction with Nd-glass lasers, but they are deficient in a number of respects. In particular, they have low non-linear optical (NLO) coefficients and are moisture sensitive. An ideal crystal would be one which is not difficult to grow, not easily damaged at moderate to high laser power levels, is phase matchable, and has a high relative NLO coefficient.
This invention is concerned with such a crystal.